There are several thousand hydrocarbon furnaces located in world refineries and petrochemical plants. In general, these furnaces vary in size and style but each contains fired heating or reaction coils most often of a serpentine configuration commonly called furnace tubes, which transport the hydrocarbon charge stock being heated and processed. During normal operation a solid carbon material, commonly referred to as coke, is formed adjacent to the inner wall of the tubing. The formation, which is a result of continuous heating of the zero velocity fluid layer immediately adjacent to the fluid boundary, grows in thickness in a continuous manner with time. Eventually, removal of the coke deposits becomes necessary due to excessive pressure drop across the tubes, reduced throughput through the tubes, or reduction in thermal efficiency below some allowable minimum.
Several methods for internal cleaning or decoking of hydrocarbon furnace tubes are currently employed, the most common of which are mechanical cleaning (commonly known as turbining), hydroblasting, and steam-air decoking.
Turbining essentially consists of cutting or reaming the coke deposits from the tube wall by passing a cutting head through each straight section. This method requires that the furnace be disassembled to the extent that the inlet and outlet of each individual straight section of tube is exposed to allow entry of the cutting head. For those furnaces of welded return bend design this means that return bends must be initially cut off and welded back in place after cleaning. Commercial sandblasting is usually employed to clean the return bends. This method has several major drawbacks, including: (1) it results in substantial downtime; (2) it is labor intensive; (3) it results in substantial tube wall wear and subsequent premature tube failure as a result of improper alignment of cutting head and furnace tube; and (4) causes severe erosion of return bends.
The second technique, known as hydroblasting, is similar to turbining except that instead of the cutting tool a hydraulic device is inserted into each tube. The device produces high pressure water jets directed normal to the tube wall which dislodge the deposit by impact. Again, this method results in substantial downtime and is labor intensive for the same reasons mentioned above. Furthermore, the high pressure water tends to dissolve sulfur initially deposited on the tube wall and results in possible sulfuric acid corrosion of the tubes in addition to creating a significant waste disposal problem.
Both of the above processes require that the furnace be cooled to near atmospheric temperature. Not only does this result in significant additional downtime, but in certain furnaces the cool down process itself can result in destruction of the furnace tubes. It is not uncommon during cool down for a furnace tube to fracture longitudinally as a result of differential thermal contraction. The heavy inner layer of coke has a significantly lower thermal expansion coefficient compared to typical tubing material and can result in circumferential thermal stresses in the tube wall in excess of its ultimate tensile strength.
Probably the most common method of decoking furnace tubes is by injecting metered amounts of steam and air into the tubes with the furnace fired. The solid coke is thus removed by a highly exothermic reaction between the solid coke and air which generates a gas-solid stream of coke particulate, CO, CO.sub.2, SO.sub.2 and NO.sub.x. The steam is used to cool the products of reaction. Process steps include: (1) removing the furnace from hydrocarbon service; (2) connecting decoking lines to the furnace; and (3) introducing steam and air to induce controlled burn out. Though furnace downtime is considerably less than the above two processes, this process can result in serious and costly furnace damage. During the process the tube skin temperature must be maintained within very narrow limits so as to both sustain the temperature required to support the reaction and yet limit the reaction temperature below the tube melting point. This highly exothermic reaction frequently results in ruptured tubes and fittings and hence costly downtime. In addition, the high temperature reaction of oxygen can leave an oxide layer on the inner tube wall which will inhibit heat transfer. Mechanical cleaning or polishing must be used to remove the deposits subsequent to steam air decoking operations. Finally, a further disadvantage of this process is that the effluent gases are highly toxic and thus create serious environmental problems, if not properly handled.